Adjustable magnetic electronic lens



AGENT Ill!! INVENTOR JAN BART LE POOLE Filed Aug. 4. 1953 2,749,464 ADJUSTABLE MAGNETIC ELECTRONIC LENS Jan Bart Le Poole, Delft, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application August 4, 1953, Serial No. 372,239

Claims priority, application Netherlands September 13, 1952 6 Claims. (Cl. 313-84) This invention relates to magnetic electronic lenses which are used in electronic microscopes or similar devices. It provides a means by which the magnification of the lens may be controlled in a rapid and simple manner and furthermore certain aberrations of the lens are avoided.

In most cases the magnification of the magnetic electronic lens is controlled by controlling the strength of the current for energising the lens. As an alternative, use is made of exchangeable pole-pieces having ditferent bores, or the distance between the poles is varied by arranging another spacer between the'pole-pieces.

For these structural changes of the lens the discharge path requires to be exhausted each time again, unless one has recourse to a complicated construction and a plurality of pole-pieces are arranged in the vacuous space, which may be moved at will to the area intended for use by means of a mechanism to be operated from the outside.

The adjustable magnetic electronic lens according to the invention does not exhibit the said disadvantages. The method of controlling this lens is not less simple than that carried out by means of the energising current but aifords with respect thereto an advantage which will be explained more fully hereinafter. Furthermore, it is suitable for lenses energised by means of a permanent magnet systerm.

The magnetic system of the magnetic electronic lens according to the invention is adapted to be displaced so as to vary the distance between the poles. This displacement requires only the. possibility of displacing one polepiece with respect to the other in the axial direction. According to the invention, the means for carrying out this displacement and fixing the pole-pieceafter displacement are provided outside the vacuous space of which the discharge path forms part. The possibility of displacing the magnetic system in this manner may be ensured-mechanically in a much simpler way than exchangeability of parts, which requires displacement at right angles to the axis.

A further possibility is to limit the vacuous space within the movable pole-piece with the use of a small tube of material having a low magnetic permeability which is provided in the bore of the said pole-piece.

A mathematic treatment of the movement of electrons in the rotation-symmetrical magnetic field of the electronic lens shows that the strength of the magnetic lens is proportional to the quadrate of the number of ampere turns. (AW) divided by the acceleration voltage (U). On this fact is based the control of the strength of the lens by varying the energising current. However, the relationship between the strength of the lens and the quotient 2,749,464 Patented June 5, 1956 is actually not linear. It appears that with increasing value of the focal length does not decrease more and more, but decreases only after an initial increase of the value (d is the aperture of the lens). This may be explained as follows.

When considering the path of an electron in the magnetic field of the lens, it appears that the path is invariably curved towards the axis. If the component of the velocity factor of the electron at right angles to the axis is initially directed away from the axis, it decreases according as the electron moves on and assumes the opposite direction if the field is sufiiciently wide. The path of the electron then approaches the axis, which is at last intersected thereby and this the sooner, according as the value is higher. If said value is caused to increase to such an extent that the intersection of the path of the electron and the axis still lies inside the field, the last part of the field causes the electron path to bend back and hence the lens becomes weaker than it would be if the magnetic field would not extend beyond the said intersection.

The curve showing the value of as a function of thus exhibits a maximum. It is preferable that the lens should be energized so as to work in this maximum, since small variations in the strength of the current for energising the lens or in the acceleration voltage for the electrons then do not aifect the strength of the lens toany appreciable extent, in other words, the lens does not exhibit any chromatic aberration in magnification. The same remark applies to the aberration referred to asdistortion (not to be confused with unequal rotation of the image). However, in this case, control of the strength of the lens by means of (strong) variation in the en ergising current is not possible without making the lenssensitive again to accidental variations in voltages or current.

When considering a number of lenses having difierent ratios between the diameter of the bore and the distance between the poles, it appears that that value of U at which the abovementioned curve exhibits a maximum is approximately the same for all lenses. The present invention utilises this circumstance. When use is made of a lens working in the maximum of the curve and when AW and hence remains unvaried and the distance between the poles is varied, the said disadvantage does ont occur at all or occurs to a much smaller extent.

Variation in image rotation which occurs upon small variation in the acceleration voltage and the energising current, respectively, may be compensated by giving different directions to the magnetic fields of the various lenses traversed successively by the electrons. Said lenses will preferably be energised by the same current, so that variations therein act upon the two lenses to the same extent.

The invention firstly applies to the projection lens of an electronic microscope. An advantage then occurring is that the current for energising the projection lens need be adjusted once only. This current may be switched on at the same time as the pump installation is put into operation, so that the coils assume their operating temperatures already during the exhaustion of the discharge path and variation in resistance due to heating does not occur afterwards. If the microscope comprises an objective having small chromatic aberration, means for stabilising the energising current and the acceleration voltage may be dispensed with.

The distance between the poles in the lens according to the invention may be varied by means of immovable arrangement of one pole-piece and slidable arrangement of the other, but also by displacing the two pole-pieces at the same time or not. The former method is simplest to carry out.

In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawing showing diagrammatically, by way of example, one embodiment of an electronic lens thus designed according to the invention in a cross-section with its plane passing through the axis.

In the figure, the reference numeral 1 indicates part of a steel tube which constitutes the body of an electronic microscope or an electron-diffraction device, that is to say that part which embraces the projection lens. Polepieces 2 and 3 are associated with the projection lens. Pole-piece 2, which is immovably arranged, has a bore d2 of 4 mms. Pole-piece 3 is slidable in the direction of the axis 4 and has a bore d3 of 8 mms. Through the last-mentioned bore extends a small brass tube 5, which constitutes the wall of the vacuous space through which the electrons are propagating.

The pole-piece is surrounded by a coil box 6 containing a magnet winding 7 which serves to magnetise the pole-pieces so as to give them opposite magnetic polarities. The magnetic flux of force traverses the iron from pole-piece 3 through a plate 8 which is secured thereto, part of tube 1 and through a partition 9 thereof, which has a funnel-like projection 10 in which the pole-piece 2 is secured.

The plate 8 has an upright edge 11 which can slide along the inner wall of tube 1. This tube has, at 12, an external thread on which a nut 13 can run. Secured to plate 8 is a catcher in the form of a tag 14 which extends to the exterior through a slot 15 provided in tube 1. The outer end of tag 14 is enclosed in a circular groove 16 of nut 13. Turning the nut has the effect of moving tag 14 through slit 15 and thus displacing pole-piece 2 in the direction of the axis without its turning about the axis. Due to the catcher not being capable of moving with respect to the nut in the axial direction, the pole-piece is fixed in any position. The distance between the poles may thus be varied between about 3 and 20 mms., so that it is possible to choose any desired magnification between two limiting values, one of which is about the 5-fold of the other.

The tube 5 which is urged from the left by a part which is not shown in the drawing, terminates in a flange 17. A rubber ring 18 is enclosed in the cavity formed by the flange, the inner wall of coil box 6 and the conical end surface of projection 10. The ring is pinched together between 17 and 10, so that it urges against the wall of the coil box and thus prevents atmospheric air from flowing along the flange into the vacuous space within tube 5 and pole-piece 2.

The sealing ring can be omitted if tube 5 and polepiece 2 are made in one piece of ferromagnetic material. However, in this case, the tube must have so small a wall thickness as to be strongly magnetically saturated during the use of the lens, so that the vacuous space between the pole-pieces is not magnetically screened thereby.

What is claimed is:

1. A magnetic electronic lens system comprising an elongated tube of low magnetic permeability defining an evacuated space and a discharge path therein, a magnetic lens including a pair of soft ferromagnetic spaced polepieces each having a central bore axially aligned and magnetically associated with the space within the tube, one of said pole-pieces being fixed and being vacuumtight coupled to said tube, the other of said pole-pieces being movable and surrounding said tube which passes through the bore thereof, and means external to the tube for axially-displacing and subsequently fixing the movable pole-piece relative to the fixed pole-piece in order to change the spacing therebetween and vary the magnification of the lens.

2. A magnetic electronic lens system comprising a thinwalled elongated tube of ferromagnetic material defining an evacuated space and a discharge path therein, a magnetic lens including a pair of soft ferromagnetic spaced pole-pieces each having a central bore axially aligned and magnetically associated with the space within the tube, one of said pole-pieces being fixed and being integrally united to said tube, the other of said pole-pieces being movable and surrounding said tube which passes through the bore thereof, and means external to the tube for axially-displacing and subsequently fixing the movable pole-piece relative to the fixed pole-piece in order to change the spacing therebetween and vary the magnification of the lens.

3. A magnetic electronic lens system as claimed in claim 1 wherein the movable pole-piece includes a catcher member projecting therefrom, the lens includes a ferromagnetic tube having an external screw thread and a longitudinal slot accommodating the catcher member, and a threaded member engages the screw thread on the tube and has a circular groove receiving the catcher member, whereby rotation of the threaded member axially displaces without rotation the movable pole-piece.

4. A magnetic electronic lens system as claimed in claim 2 wherein the movable pole-piece includes a catcher member projecting therefrom, the lens includes a ferromagnetic tube having an external screw thread and a longitudinal slot accommodating the catcher member, and a threaded member engages the screw thread on the tube and has a circular groove receiving the catcher member, whereby rotation of the threaded member axially displaces without rotation the movable pole-piece.

5. A magnetic electronic lens system comprising means defining an evacuated space and a discharge path therein, a magnetic lens including a pair of soft ferromagnetic spaced pole-pieces axially aligned and magnetically associated with said discharge path, means for energizing the magnetic lens to a value at which its focal length is minimized, and means external to the evacuated space for axially displacing and subsequently fixing one of the polepieces relative to the other in order to change the spacing therebetween and thus vary the magnification of the lens.

6. A magnetic electronic lens system as set forth in claim 5 wherein the energizing means for the lens provides constant energization independent of the magnification of the lens.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Mynall Oct. 22, 1940 Hillier Sept. 8, 1942 5 Hillier Feb. 20, 1945 Hillier et a1. Apr. 1, 1947 Hillier May 3, 1949 6 Linder -1. Sept. 11, 1951 Van Gilder Apr. 22, 1952 Wolff Dec. 30, 1952 Shaw et al. July 21, 1953 Reisner et a1 May 18, 1954 FOREIGN PATENTS Great Britain Sept. 13, 1947 

